British companies will be £10bn worse off after new pension accounting rules take effect, according to accountants KPMG.

Plans by the International Accounting Standards Board to introduce "son of FRS17" will hit the profits of all companies with final salary retirement schemes, the pension experts said.

Some pension analysts said the new rules would bring greater transparency to pension accounting and prevent companies from artificially inflating their profits with gains made by investments in their occupational retirement schemes.

Independent pension consultant John Ralfe said his analysis showed BT would suffer a £750m hit on its annual profits if the rule becomes binding on companies. Ralfe said the reform was "unfinished business" from FRS17.

At the moment companies can credit the difference between what they lose on interest payments on pension assets against what they expect to gain from investment returns. Typically a 5% interest bill is more than offset by a 7% return on assets. Ralfe said an example using BT's pension fund shows it gains by £500m under the existing rules and loses £250m under the new rules, leaving a net loss of £750m.

"This is another step in the right direction of transparent pension accounting and reduces the scope for people to play ducks and drakes," he said.

Most companies closed their final salary schemes to new members after FRS17 was introduced almost 10 years ago. For the first time employers, which were mostly invested in equities, were forced to give a snapshot of their pension assets values. In the stock market crash of 2003 huge deficits in scheme funds appeared and finance directors sought ways to minimise the damage.

In the wake of the 2008 stock market dive many companies closed their schemes to existing members.

The latest ruling is expected to encourage more employers to shut their schemes altogether. Sarah Abram, of Aon Consulting said: "For most businesses, our analysis shows that the changes will translate into both lower profits and a worse balance sheet position."